There is a long recognized and continuing need for cost effective renewable energy sources. With this goal, significant efforts have been made to develop cost effective solar powered generators to harvest solar energy. The main focus of these efforts have been to make high efficiency low cost solar panels.
Solar panels are photovoltaic devices designed to convert solar energy directly to electricity. Basic solar panel technology is based on p-n junctions. The difference in charge carrier concentration between p- and n-doped regions of a semiconductor material cause charge carriers to diffuse, thereby creating a static electric field within the semiconductor. The semiconductor has a band gap energy which is the energy difference between the minimum of its conduction band maximum of its valence band. Many semiconductors the band gap energy that lies within limits of the solar radiation spectrum. Photons with energy greater than the band gap energy can be absorbed by the semiconductor and raise charge carriers from its valance band to its conduction band. The excited carriers flow as a result of the electric field and provide electrical power.
Solar panels in current use can be broadly divided into crystalline silicon and thin film technologies. Crystalline silicon is a relatively poor absorber of light and requires a comparatively large thickness (several hundred microns) of material in comparison to materials such as Cadmium Telluride (CdTe) and Gallium Arsenide (GaAs) used in thin film technologies. Presently, crystalline silicon solar panels provide higher efficiencies than thin film solar panel, but are more expensive to make. Good conversion efficiencies for solar panels commercially available at this time are in the range from 14-19%. Higher conversion efficiencies are possible.
A maximum efficiency for converting un-concentrated solar radiation into electrical energy using a single junction solar panel at room temperature is about 31% according to the well known Shockley-Queissar limit. This limit takes into account a thermodynamically unavoidable rate of carrier recombination and a mismatch between the band gap energy of the semiconductor and the solar energy spectrum.
The mismatch relates to the quantization of energy in light. Wavelengths of light with energy below the band gap energy cannot excite charge carriers. Wavelengths of light with energy above the band gap energy can excite carriers, but the energy in excess of the band gap energy is rapidly converted to heat. Band gap energies around 1.3 eV provide the highest theoretical efficiency for a single-junction solar panel at room temperature.
The Shockley-Queissar limit for a single-junction solar panel can be exceeded by providing multiple junctions. A typical multi-junction solar panel comprises a layered stack of two or more semiconductor materials having different band-gap energies. The uppermost layer has the highest band gap energy. Ideally, the uppermost layer absorbs the portion of the spectrum with energy equal to or greater than the upper layer's band gap energy while passing longer wavelengths for use by the layers beneath.
Optical transparency for the layered structure generally requires that all layers have similar crystal structure and lattice constants. A lattice constant describes the spacing of the atom locations in a crystal structure. Mismatch in the lattice constants between different layers tends to creates dislocations and significantly deteriorates the efficiency of a multi-junction solar panel.
While the choice of materials for multi-junction solar panels is constrained, many suitable combination have been found and shown to outperform single junction cells. By suitably dividing the absorption spectrum, excellent results have been obtained with two, three, and four junction cells. For example, a two junction cell comprising InGaP (1.9 eV) and GaAs (1.4 eV) held a record efficiency near 30% in the 1990's. A three junction cells comprising GaInP (1.85 eV), GaAs-layer (1.42 eV) and Ge (0.67 eV) has been used to demonstrate efficiencies near 40%.
Another way to improve solar panel efficiencies is by concentrating sunlight onto the solar panel surface. Aside from the obvious benefit of providing more light per unit area, the direct light provided by a concentrator (as compared to the diffuse light received by a panel directly exposed to the sun) allows for a higher efficiency. 41% is the theoretical limit for a single junction cell and 55% for a two junction cell. For direct sunlight, the optimal bandgap energy at room temperature is 1.1 eV. For a two junction cell in the standard series configuration, a 0.77 eV, 1.55 eV pairing is the approximate optimum. For three junctions, 0.61 eV, 1.15 eV, and 1.82 eV approximates the ideal as reported by M. A. Green in. Third-Generation Photovoltaics: Advanced Solar Energy Conversion, pp 60-63 (Springer: Heidelberg, 2003).
A further improvement to enhancing the electrical conversion efficiency involves deriving electrical energy from the excess energy absorbed when an electron is excited by a photon with energy in excess of the band gap energy. Initially, this energy is retained by the carriers, resulting in “hot carriers”. There are two fundamental ways to use the hot carriers for enhancing the efficiency of electrical energy production. One way produces an enhanced voltage and the other produces an enhanced current. The former requires that the carriers be extracted before they cool, while the latter requires that hot carriers having sufficient energy to produce a second electron-hole pair through impact ionization. For either process to be effective, it must be carried out at a rate competitive with the rate of carrier cooling, which is itself very fast.
The rate of carrier cooling can be greatly reduced by producing the carriers within a nanocomposite material that alters the relaxation dynamics through quantum effects. Nanocomposite materials include quantum wells, quantum wires, and quantum dots. These structures confine the carriers to regions of space that are smaller than or comparable to the carrier's deBroglie wavelength or to the Bohr radius of excitons in the semiconductor bulk. Quantum dots are most effective in this regard.
Quantum dots consisting of very small crystals of one semiconductor (e.g. Indium Gallium Arsenide) within a matrix of another semi-conductor (e.g., Gallium Arsenide) can slow carrier cooling to the point where impact ionization becomes significant. Impact ionization when a hot carrier gives up some of its energy to excite a second carrier from the valence band to the conduction band while itself retains sufficient energy to remain in the conduction band. Impact ionization can also be achieved by quantum dots consisting of very small semiconductor crystals dispersed in an organic semiconductor polymer matrix.
Hot carrier extraction can be achieved by ordering the quantum dots in closely spaced three-dimensional array with sufficiently close spacing for strong electronic coupling and the formation of mini-bands to occur. The mini-bands allow long-range electron transport. The mini-bands provide fast enough transport for the hot carrier current to be drawn off at a potential above the normal conduction band potential. To understand this mechanism, it may help to note that the hot carrier energy spreads among all the carriers in the conduction band on a shorter time scale than the timescale on which the energy spreads towards thermal equilibrium in other ways. Thus the entire carrier stream is “hot”.
To avoid confusion with the above mechanisms, it is worth noting there is another use for quantum wells in enhancing solar panel efficiencies. Quantum wells can be used to adjust and finely tune the band gap energies of the semi-conductors into which they are incorporated. This allow semi-conductor band gap energies to be adapted to better match the solar spectrum and provides flexibility in selecting materials.
Still further, nanocrystals within a semiconductor composite have highly size dependent band gap energies. These can be used to make available to charge carriers energy states intermediate the valance and conduction bands of the matrix materials. These intermediate bands allow the composite to achieve electrical conversion of photons with energy below the band gap energies of the matrix semiconductors through a two step process of exciting charges from a valance band to the intermediate bands and from the intermediate bands to a conduction band.
Many of the foregoing structural enhancements are only economical in conjunction with solar concentration. Commercially available solar concentrators provide solar energy with concentrations of 500. While such high concentration justifies the use of highly engineered semiconductor materials it introduces the problem of managing intense heat. Heating is very detrimental to solar panel performance.
The theoretical maximum efficiencies quoted above all diminish with increasing temperatures. All solar panels undergo diminishing efficiency with increasing temperature. According to the National Aeronautics and Space Administration (NASA), as reported in U.S. Pat. No. 7,148,417, a typical silicon solar panel loses about 0.45% power per degree centigrade of increasing temperature. Above 250° C., silicon solar panels produce essentially no power. GaAs solar panels fare somewhat better, losing only about 0.21% power per degree Celsius. Multi-junction thin film solar panels generally fared even worse because the layer thickness are generally carefully matched to equalize currents produced by each layer. Even a 5% mismatch can severely disrupt the multi-Junction solar panel's operation. M. A. Green in Third-Generation Photovoltaics: Advanced Solar Energy Conversion, p, 63 (Springer: Heidelberg, 2003). The routine solution of this problem is to provide cooling.
Solar panels have been used to provide hot water for domestic use in addition to electricity. As noted by U.S. Pat. No. 2004/0055631, using the solar panel in this manner requires operating the solar panel at a temperature of at least about 60° C., which significantly compromises the cell's electrical production efficiency. The solution proposed by that application is make the solar panel to a portion of the solar energy spectrum with energy below a semiconductor's band-gap energy. The solar panel is insulated from the heating elements, which utilize a portion of the solar energy spectrum that could not be converted to solar panel. The solution is said to be more space efficient than the alternative of using separate solar energy collectors for electricity production and water heating. Another way to go generate hot water is to draw heat from the solar collection system. When high degrees of solar concentration are used, the waste heat can be considerable.